In many modern CMOS imagers employing pixel arrays the optical integration time is controlled by the method known as “rolling shutter”. This approach utilizes at least two pointers, Reset and Read, that continuously move through the pixel array image frame from top to bottom jumping from line to line at line-time intervals. First, the Reset pointer 105 starts the integration for all pixels in the line. Some time later the Read pointer 110 reaches the same line and initiates signal readout. This is illustrated in FIG. 1, which depicts a pixel array. The distance in lines between the two pointers is referred to as shutter width 115. Shutter width multiplied by the line time gives the duration of the optical integration time.
The use of “rolling shutter” approach allows an equal optical integration time for all pixels in an image frame to be achieved. However, this optical integration does not happen for all pixels simultaneously with the actual time interval used for integration dependent on the vertical position of the pixel in an image frame. Under certain conditions described below, this method of integration time control leads to the possibility of flicker or “running bands” in the image frame.
Conventional imagers avoid flicker either by off-line specification of the AC frequency or by auto-detection techniques that rely on detecting temporal flicker frequency, typically in the middle of an image frame.